The Mole
Chemistry 2nd semester
Name
Period
Teacher
Learning goals for Moles (February)
I will be able to
1. Define and the following terms: average atomic mass, formula mass, percent composition, mole, Avogadro's
number, relative atomic mass, molar mass, empirical formula, and molecular formula Demonstrate that one mole of
an element is equal to its relative atomic mass in grams.
2. Calculate the average atomic mass of an element
3. Calculate the formula mass for a compound
4. Demonstrate that one mole of a compound is equal to its molar mass in grams.
5. Explain why Avogadro's number has the value 6.02 x 10 23
6. Demonstrate that one mole of any substance (element, diatomic element, compound) is equal to Avogadro's
number of particles.
7. Interconvert between mass, moles, and numbers of particles.
8. Calculate the molar mass of a compound.
9. Calculate the empirical formula of a compound from the percent composition.
10. Calculate the empirical formula of a compound from the mass of each element in the compound.
11. Calculate the molecular formula of a compound from the empirical forumla and the molar mass.
12. Calculate the molecular formula of a compound from the percent composition and the molar mass.
13. Calculate the molecular formula of a compound from the mass of each element in the compound and the molar
mass.
14. Calculate the percent composition of a compound from the molecular formula.
15. Calculate the percent composition of a compound from the masses of elements in the compound.
Reviewing Elements, Compounds, and Counting atoms
Write the letter E in the space if the substance is an ELEMENT.
Write the letter C in the space if the substance is a COMPOUND.
1.
H2O
2.
NaCl
3.
Cl2
4.
Fr
5.
Al2O3
6.
Ir
7.
Br2
8.
Ca(HCO3)2
9.
(NH4)2O
10.
N2
11.
Mg(OH)2
12.
As
Determine and write the number of atoms of each element in the following compounds.
13. NaHCO3
14. (NH4)2O
Number of Na atoms
Number of C atoms
Number of H atoms
Number of O atoms
Number of H atoms
Number of N atoms
Number of O atoms
15. Ca(HCO3)2 Number of Ca atoms
Number of H atoms
Number of C atoms
Number of O atoms
Average Atomic Mass
By the end of this activity you should be able to define and calculate average atomic mass for any element given
the mass of its isotopes and percent composition.
You might remember from last semester that we defined the average atomic mass, or you might not. Here is a quick review
of atomic structure. Taking the information of a given element from the periodic table we find:
19
Atomic number is the number of protons in the atom
K
39.098
Average atomic mass is the mass of an element calculated using the
relative abundance of naturally-occurring isotopes for that element.
The mass of an atom, as you should remember, is given by the sum of the protons and neutrons.
Isotopes are atoms of the same atom that have different number of neutrons. Another way of saying the same thing would
be atoms with the same atomic number but different atomic mass.
Today you will practice differentiating between atomic mass and average atomic mass of objects that will simulate being
atoms.
Materials:


“atoms”: paper clips and pipets of different sizes
Balance
You will be finding the mass of different objects. Include the appropriate unit for every measurement you make
Procedure
1. Mass independently three small paper clips and three large paper clips and fill in the table
Mass in grams Clip #2
Mass in grams Clip #1
Mass in grams Clip #3
Small paper clip
Large paper clip
a. What would you say is the mass of small clip, and how consistent is that mass among your small clips?
b. What would you say is the mass of small clip, and how consistent is that mass among your large clips?
2. Find the mass of 9 small paper clips and 1 large paper clip by placing all 10 clips on the balance: ______________
a. Calculate the average of one clip among the 10 you massed _____________________
3. Find the mass of 1 small paper clip and 9 large paper clip by placing all 10 clips on the balance: ______________
a. Calculate the average of one clip among the 10 you massed _____________________
4. If the paper clips were atoms of the same element, how many isotopes would be represented by the 10 paper clips in the two
previous cases? _______
Let us try it again with pipettes this time.
5. Mass independently three small pipets and three large pipets and fill in the table
Mass in grams pipette #2
Mass in grams pipette #1
Mass in grams pipette #3
Small pipette
Large pipette
a. What would you say is the mass of small clip, and how consistent is that mass among your small pipettes?
b. What would you say is the mass of large clip, and how consistent is that mass among your large pipettes?
6. Find the mass of 7 small pipettes and 3 large pipettes by placing all 10 pipettes on the balance: ______________
a. Calculate the average of one pipettes among the 10 you massed _____________________
We can calculate the percentage (%) of a given size of pipettes in the sample we use the following formula:
%=
# 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠 𝑜𝑓 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑠𝑖𝑧𝑒
𝑥 100.
𝑡𝑜𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
The calculation of the percentage of small pipettes becomes:
%=
# 𝑠𝑚𝑎𝑙𝑙 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
7 𝑠𝑚𝑎𝑙𝑙 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
𝑥 100 =
𝑥 100 = 70%
𝑡𝑜𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
10 𝑡𝑜𝑡𝑎𝑙 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
b. Calculate the percentage of large pipettes: ____________________________
We can calculate the average atomic mass of the pipettes using the atomic mass for each size of the pipette and their
percentage. Imagine a set of 8 small pipettes with a mass of 0.9 g and 2 large pipettes with a mass of 1.6 g. What would
the average percentage be?
I.
II.
Calculate percentage for each type of pipette:
% 𝑠𝑚𝑎𝑙𝑙 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠 =
8 𝑠𝑚𝑎𝑙𝑙 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
10 𝑡𝑜𝑡𝑎𝑙 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
𝑥 100 = 80%
% 𝑙𝑎𝑟𝑔𝑒 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠 =
2 𝑠𝑚𝑎𝑙𝑙 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
10 𝑡𝑜𝑡𝑎𝑙 𝑝𝑖𝑝𝑒𝑡𝑡𝑒𝑠
𝑥 100 = 20%
Calculate the average atomic mass by multiplying the percentage composition for each pipette times its
atomic mass, adding the products of the different pipettes and dividing by 100.
Average atomic 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 =
=
(% 𝑠𝑚𝑎𝑙𝑙 ∗𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑠𝑚𝑎𝑙𝑙)+(% 𝑙𝑎𝑟𝑔𝑒 ∗𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑙𝑎𝑟𝑔𝑒)
(80∗0.9 𝑔)+(20∗1.6𝑔)
100
100
=
72𝑔+32
100
=
104𝑔
100
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 = 1.04𝑔
c. Calculate the average atomic mass using percentages and atomic mass: _______________
d. Compare the answers you gave to questions 6a. and 6c. Are the values similar or different? Explain your answer in
complete sentences
7. Find the mass of 6 small pipettes and 4 large pipettes by placing all 10 pipettes on the balance: ______________
a. Calculate the average of one pipette among the 10 you massed _____________________
b. Calculate the percentage composition for each pipette:
c. Calculate the average atomic mass using percentage and atomic mass
d. Compare the answers you gave to questions 7a. and 7c. Are the values similar or different?
8. If the pipettes were atoms of the same element, how many isotopes would be represented by the 10 pipettes in the two
previous cases? _________
9. Describe the difference between the atomic mass of an atom and the average atomic mass of an element:
Propose a method by which you could find the mass of a compound made put of three paper clips and two pipettes. Here is
the formula:
2
3
To calculate the mass of a compound made of two pipettes and three paperclips I would …
As a refresher, calculate the oxidation number of the pipette and paper clip given the formula above:
Oxidation number of the pipette:
Oxidation number of the paper clip:
Valence electrons of the pipette:
Valence electrons of the paper clip
Can you define average atomic mass? _________
Can you calculate the percent composition of an isotope given the number of atoms for each isotope? _______
Can you calculate the average atomic mass for any element given the atomic mass of its isotopes and percent composition ?
Let your teacher know if you answer no to any of these three questions
Assignment Chem 10
Name
Average Atomic Mass
Reference Reading: Modern Chemistry pg. 77 and 78.
Average Atomic mass or atomic weight is the average mass of atoms of an element, calculated using
the relative abundance of isotopes in a naturally-occurring element.
1. 100 fifth grades students were weighed at Winnequah and their weights recorded. 77 of the students each
weighed 85 lb, while the remaining 23 students each weighed 87 lbs. What is the average weight of these
100 fifth graders? Show your work.
2. If the atomic mass of an atom is simply the sum of the number of protons and neutrons, explain why the
atomic mass of any particular element on the periodic table is a decimal.
3. Without doing any math, are there more Br-79 atoms or more Br-80 atoms on earth? (Hint: look at the
periodic table.) Explain your answer.
4. Oxygen has 3 isotopes, O-16, O-17, O-18. Again, without doing any math which isotope is the most
common? Explain your answer.
For questions 5-12 you must do 5 pts of work. Each Question shows the point value.
5. (1 pt) What is average atomic mass of Lithium if 6.00% exists as Li-6 and 94.00% exists as Li-7?
6. (1 pt) A random sample of the element Boron is found to be 20.0% B-10, and 80.0% B-11. What is boron’s
average atomic mass?
7. (1 pt) In a sample of 200 Chlorine atoms, it is found that 110 are Cl-35 and 90 are Cl-36. What is the
average atomic mass of Chlorine?
8. (1 pt) Rubidium has two common isotopes, Rb-85 and Rb-87. If the abundance of Rb-85 is 77% and the
abundance of Rb-87 is 23%, what is the average atomic mass of rubidium?
9. (1 pt) Neon has two major isotopes, Neon-20 and Neon-22. Out of every 250 neon atoms, 225 will be
Neon-20, and 25 will be Neon-22. What is the average atomic mass of Neon?
10. (2 pts) Magnesium has three naturally occurring isotopes. 78.70% of Magnesium atoms exist as Mg-24,
10.03% exist as Mg-25,and 11.17% exist as Mg-26. What is the average atomic mass of Magnesium?
11. (2 pts) Out of 500 silicon atoms, 460 are Si-28, 25 are Si-29 and 15 are Si-30. What is the average atomic
mass?
12. (3 pts) What is the average atomic mass of hafnium if, out of every 100 atoms, 5 have a mass of 176 amu,
19 have a mass of 177 amu, 27 have a mass of 178 amu, 14 have a mass of 179 amu, and 35 have a
mass of 180 amu? Make sure to show your work.
Formula Mass
By the end of today’s lesson you should be able to define and calculate the formula mass for any compound
Let us start with the idea of Formula Mass with non-chemical examples…
1. Two classmates were complaining about how heavy their backpacks were each day when they went home to study. They
decided to weigh their items on a scale and determine who had the heavier backpack.
Naomi is carrying 2 textbooks, 4 notebooks, and a pair of basketball shoes.
Chris is carrying 2 textbooks, 2 notebooks, and a trumpet.
Item
Weight (lbs)
Determine the weight of each bag.
Textbook
5
Notebook
0.4
Basketball shoe (1)
1
Trumpet
3.5
a. Determine the total weight of each item in Naomi’s backpack. Multiply the quantity by each item’s weight from the table.
Item Quantity
Textbooks
Weight of Item
X
=
Notebooks
X
=
Basketball Shoes X
Total Weight for Each Item
=
Now, add up the total weight for each item to get the weight of Namoi’s backpack.
b. Use the same steps to determine the weight of the items in Chris’ backpack.
Item Quantity
Weight of Item
Total Weight for Each Item
Textbooks
X
=
Notebooks
X
=
Shoes
X
=
Now, add up the total weights for each item to get the weight of Chris’ backpack.
c. Which backpack weighs more?
2. The fast food restaurants in the area (McDonalds, Burger King, and Wendy’s) are holding a “Family Pack Sale”. Each food
chain had the following Family Packs at a discounted price. Determine the amount of calories in each Family Pack.
The menu shows the calorie amount for ONE item. Use the steps considered in example 1 to fill out the chart.
(Note: It is recommended that the average person consumes about 2,000 calories daily).
A.
McDonalds’ Family Pack includes:
1 Big Mac
2 Chicken Nugget orders (10pc each)
4 Large Fries
4 Large Sodas
McDonalds Menu
Item
Calories
Big Mac
540
Chicken Nuggets (10pc) 470
Large Fries
500
Large Soda
310
Total Calories per
McDonalds Family Pack =___________________________
Wendy’s Menu
Item
B. Wendy’s Family Pack Includes:
2 Triple W’s
1 Chicken Sandwich
2 Large Fries
3 Large Sodas
Calculation
Calories per Menu
Item in Family PK
Calories
Calculation
Calories per Item in
Family PK
The Triple “W”
1060
Crispy Chicken
700
Sandwich
Large Fries
500
Large Soda
310
Total Calories per
McDonalds Family Pack =___________________________
Item
Burger King Menu
Calories
Triple Whopper/Cheese
BK Quad Stacker
Large Onion Rings
Large Soda
1230
830
500
310
Calculation
Calories per Item in
Family PK
C. Burger King’s Family Pack Includes:
2 Triple Whoppers w/ cheese
2 BK Quad Stackers
3 Large Onion Rings
4 Large Sodas
Total Calories per
McDonalds Family Pack =___________________________
In the previous activity, we asked you to propose a method to find the mass of a compound using pipettes and paper clips
for the following formula:
2
3
In the above examples, we illustrated methods with burgers, fries, and backpacks. What does this have to do with
chemistry?
The formula mass of any molecule, formula unit, or ion is the sum of the average atomic masses of all atoms represented
in its formula.
We have determined how the average atomic masses are calculated on the periodic table for each element. We can use
this information to find the mass of a substance with more than one element.
3. Calculate the formula mass for Sodium Chloride
a. Determine the number of atoms of each element.
NaCl
Na =
Cl =
b. Multiply the number of atoms of the element by the average atomic mass taken from the periodic table.
REMEMBER TO WRITE THE CORRECT UNITS ALL ALONG YOUR CALCULATIONS!
Na:
Cl:
c. Add the totals to get the total FORMULA MASS
Include the correct units!
4. Calculate the formula mass for Carbon Dioxide
CO2
a. Determine the number of atoms of each element.
C=
O=
b. Multiply the number of atoms of the element by the average atomic mass on the periodic table.
C:
O:
c. Calculate the formula mass for Add the totals to get the total FORMULA MASS
5. Magnesium Hydroxide
Mg(OH)2
a. Determine the number of atoms of each element.
Mg=
O=
H=
b. Multiply the number of atoms of the element by the average atomic mass on the periodic table.
Mg:
O:
H:
c. Add the totals to get the total FORMULA MASS
Can you define formula mass? _____________
Can you calculate the formula mass of any compound? ________________
Let your teacher know if you answer no to any of the two questions
Assignment Chem 10
Name
Formula Mass
Reference Reading: Modern Chemistry pg. 225 and 226.
The Formula Mass of a molecule is the sum of the atomic weights of the atoms in a compound.
1. Explain what formula mass is? What analogy was used in class to help you understand this concept?
For questions 2-9 you must do 5 pts of work. Each Question shows the point value.
2. (1 pt) The formula for sodium nitrate is NaNO3. Write the number of atoms of each element, along with their
average atomic masses. Determine the formula mass for sodium nitrate.
3. (1 pt) The formula for aluminum oxide is Al2O3. Write the number of atoms of each element, along with their
average atomic masses. Determine the formula mass for calcium bicarbonate.
4. (1 pt) The formula for magnesium hydroxide is Mg(OH)2. Write the number of atoms of each element, along
with their average atomic masses. Determine the formula mass for calcium bicarbonate.
5. (1 pt) The formula for iron III carbonate is Fe2(CO3)3. Write the number of atoms of each element, along with
their average atomic masses. Determine the formula mass for calcium bicarbonate.
6. (1 pt) The formula for germanium phosphate is Ge3(PO4)2. Write the number of atoms of each element, along
with their average atomic masses. Determine the formula mass for calcium bicarbonate.
7. (2 pts) Write the formula for calcium bicarbonate. Write the number of atoms of each element, along with
their average atomic masses. Determine the formula mass for calcium bicarbonate.
8. (2 pts) Write the formula for ammonium acetate. Write the number of atoms of each element, along with their
average atomic masses. Determine the formula mass for calcium bicarbonate.
9. (3 pts) A molecule of an unknown compound contains aluminum, chromium, and oxygen. It was determined
that 156 amu of the formula mass is from chromium and 336 amu is from oxygen. It was further determined
that the compound has a polyatomic ion which is made of chromium and oxygen Using this information,
determine the formula mass of this compound and the formula of this compound.